Acetaminophen (paracetamol) is a para-aminophenol antipyretic-analgesic agent, and is widely used for pain relief of headache, myalgia, menstrual pain, toothache and the like, and for pain relief and fever reduction in acute upper airway inflammation etc. Generally, acetaminophen is orally administered in the form of a tablet, a syrup or the like.
However, acetaminophen in an aqueous solution is unstable and susceptible to hydrolysis, which results in orange to brown discoloration.
So far, there have been various attempts to improve the stability of acetaminophen in an aqueous solution. For example, Patent Literature 1 discloses that addition of pyrosulfites such as sodium pyrosulfite to an aqueous composition containing acetaminophen improves the stability of acetaminophen.
Patent Literature 2 discloses that discoloration of an aqueous solution containing acetaminophen can be prevented by addition of polyols as a free radical scavenger, such as mannitol, sorbitol and inositol, to the aqueous solution.
However, the methods of Patent Literature 1 and 2 cannot stabilize acetaminophen to a practically sufficient degree.
Aside from these disclosures, Patent Literature 3 discloses that acetaminophen blended with glycine has an improved antipyretic action.
Patent Literature 4 discloses that aminoacetic acid (glycine) improves the antipyretic-analgesic effect of ibuprofen, a nonsteroidal antipyretic-analgesic agent.